A plasma display device (“PDP”), which displays an image by causing a number of fine cells to emit light by themselves utilizing an electric discharge phenomenon, has excellent features that cannot be realized by conventional display devices, such as a large and thin size, light weight, and flat shape, which is becoming widespread.
Most of conventional plasma display devices employ cells having a straight structure in which ribs are formed only in the vertical direction on the display surface. However, for efficiently introducing light to the front of the plasma display device, there have recently been developed cells having a waffle structure in which ribs are formed not only in the vertical direction but also in the horizontal direction. With the cells having such a waffle structure, leakage of light from the adjacent cells is prevented, enabling an extremely efficient introduction of light to the front.
FIG. 1 is an exploded perspective view of an essential portion of a plasma display device having waffle cells. The plasma display device includes a front plate 1 having combined electrodes 11 formed in parallel to one another wherein each combined electrode is made of a transparent electrode 110 and a bus electrode 112, and a back plate 2 having address electrodes 21 formed in parallel to one another in the cross direction with respect to the combined electrodes 11. The front plate 1 and the back plate 2 are disposed so as to face each other and unified to constitute a display element. The front plate 1 has a transparent glass substrate 10 as a display plane, and the combined electrodes 11 are disposed on the inner side of the glass substrate 10, namely, on the side thereof facing the back plate 2. A dielectric layer 12 is formed so as to cover the combined electrodes 11, and a patterned spacer layer 16 is provided on the dielectric layer 12. A protective layer 19 made of, for example, MgO is formed on the surface of the dielectric layer 12 and the spacer layer 16. On the other hand, the back plate 2 has a substrate 20, which is provided with the address electrodes 21 disposed on a side of the substrate 20 facing the front plate 1. A dielectric layer 22 is formed so as to cover the address electrodes 21, and the light emitting portions are formed on the dielectric layer 22 as described below.
The light emitting portions consist of a number of cells each of which is located in a space at which the combined electrode 11 crosses the address electrode 21. Each cell is confined by ribs 24 formed on the dielectric layer 22 along the vertical and horizontal directions of the display (i.e., the direction indicated by arrows V and H, respectively shown in FIG. 1). A fluorescent layer 26 is provided so as to cover the sidewall of the rib 24 and the surface of the dielectric layer 22 in the rib, that is, the inner wall and bottom of each cell. In the plasma display device, a predetermined voltage from an alternating power source is applied to the combined electrodes of the front plate to form an electric field between the electrodes, so that an electric discharge occurs in the cells. This discharge results in generation of an ultraviolet light, which further causes light emission of the fluorescent layer 26.
FIG. 2 is a perspective view of the front plate 1 of the plasma display device having waffle cells, as seen from the back plate side. FIG. 3 is a cross-sectional view of the plasma display device having waffle cells. As shown in FIG. 2, in the plasma display device having the waffle structure, a number of spacer layers 16 are provided on the dielectric layer 12 so that they are arranged in a form of equally spaced lines. As shown in FIG. 3, in the front plate 1, the spacer layer 16 is in contact with the rib 24. A gap X is thus formed at the upper portion of each cell surrounded by the rib 24, and a rare gas can be introduced to each cell through the gap X.
A process for producing such a front plate is roughly classified into a production process utilizing a screen printing method and a production process utilizing a photolithography method.
In the production process utilizing a screen printing method, a glass paste layer is formed on the glass substrate 10 and baked at 500 to 700° C. to form the dielectric layer 12. On the dielectric layer 12, a glass paste composition is then stacked in a patterned form by screen printing, and further baked at 500 to 700° C. to form the spacer layer 16.
However, the production process utilizing the screen printing method has problems of the cost for production due to the essential two baking steps, as well as a poor precision of the pattern alignment.
Referring to FIG. 10, the production process utilizing a photolithography method is then described. On the glass substrate 10 are formed an unbaked dielectric layer 12A consisting of a non-photosensitive glass paste layer, as well as a photosensitive, unexposed unbaked spacer material layer 16A consisting of a photosensitive glass paste layer. The spacer material layer 16A is then irradiated with, e.g., an ultraviolet light through a photomask 3 (FIG. 10A). The layer is then developed so that a resist pattern 16A′ appears (FIG. 10B). The resultant product is baked at 500 to 700° C. to form the dielectric layer 12 and the spacer layer 16 simultaneously (FIG. 10C).
In the production process utilizing a photolithography method, the dielectric layer 12 and the spacer layer 16 can be baked at the same time in a single baking operation, and therefore the cost for production can be advantageously lowered, as compared to the cost for the production process utilizing a screen printing method.
However, upon appearance of the resist pattern after the development treatment in such a production process, the spacer material often remains in regions other than the regions in which the material should be left as a spacer layer (see FIG. 10B). Although the spacer material residue A remaining in the region that has been subjected to the removing development (concave region) becomes somehow flat due to melting of the glass frit component in the baking treatment, it causes unevenness of the exposed surface of the dielectric layer 12, leading to a problem that the thickness of the dielectric layer 12 between the spacer layers 16 becomes ununiform (see FIG. 10C).
As shown in FIG. 3, in the plasma display device, the light emitting portions are disposed between the spacer layers 16. When the thickness of the dielectric layer 12 at that portion is not uniform, the light transmittance or electric discharge properties become ununiform, which can be one of the reason for causing distortion in the image.